Model - is the model of your YSL bag desired or rare? Different models have different demand. This factors in whether or not there are certain styles and looks trending.
Material - Naturally, the more expensive materials will increase its price, such as leather as oppose to fabric or synthetic.
Condition - It goes without saying, the better condition your YSL bag is in, the more valuable it will be.
Generally trend or limited edition YSL bags will fetch the highest prices due to demand and availability.